I think you're being a bit unreasonable, JRF. First of all, CO doesn't service ASE except by offering codeshares on NW flights. 2nd, CO only operates one flight per day out of AMS (they don't code share on the NW flights to AMS). This flight does not arrive in EWR until 3:25 which allows a connection to only one code-share flight combination to get to ASE. This means there is only one flight combination per day that goes AMS-ASE on CO and two of the flights are codeshares, meaning that seats are likely to be limited, particularly in lower fare classes.

It is not reasonable to expect the CO reps to be good a putting together itineraries involving a bunch of non-CO flights. This is a route which simply is not well served by CO, so I don't see a point in complaining that their agents have trouble finding you a low fare on such a route.